Compound semiconductors, such as the III-V material GaAs, offer many advantages over silicon for advanced semiconductor devices. GaAs in particular has been developed for high speed integrated circuits. However, GaAs, as well as other III-V and II-VI semiconductors, suffer from poor electronic surface quality. That is, the surface provides electrical traps and defects which remove carriers from the underlying semiconductor. The surface problem is much less severe in silicon since the oxide SiO.sub.2 provides a good passivating surface to Si. It is known that the quality of surface passivation can be quantified in terms of the surface recombination velocity S.sub.o. The lower the value of S.sub.o, the better is the passivation. Thermally oxidized silicon shows a surface recombination velocity of .about.100 cm/s while GaAs shows a value no better than .about.10.sup.6 cm/s.
Two of the three present inventors described the passivation problem in their U.S. Pat. No. 4,751,200, issuing in the names of Gmitter et al. They were able to passivate GaAs by applying a solution of Na.sub.2 S.9H.sub.2 O to a cleaned GaAs substrate and to thereby form a sodium sulfide passivating film thereupon. The film was about 0.5 .mu.m thick. Thereby, the surface recombination velocity of the GaAs was reduced by a factor 60 or 100, that is S.sub.o .about.10.sup.4 cm/sec.
Commonly assigned U.S. Pat. No. 4,751,201, issued in the names of Nottenburg et al, disclosed how the method of Gmitter et al could be applied to actual electronic devices, such as a vertical GaAs transistor.
These results of Gmitter et al were a distinct advance over the prior art. Nonetheless, the electrical passivation of GaAs still was poor compared to Si. More importantly, it was discovered that the sodium sulfide passivation of GaAs was not stable. Although S.sub.o was initially measured at around 10.sup.4 cm/sec, it was discovered that S.sub.o thereafter began to and continued to increase to unacceptable values. Therefore, this prior work did not provide a satisfactory commercial process for passivating GaAs.
Yablonovitch et al in a technical article entitled "Nearly ideal electronic properties of sulfide coated GaAs surfaces" appearing in Applied Physics Letters, volume 51, 1987 at pages 439-441 disclose both the subject matter of the Gmitter et al patent but also the use of (NH.sub.4).sub.2 S as a passivating material. They further report on wafer treatment in an aqueous base. Sandroff et al have disclosed a similar passivating technique in a technical article entitled "Electronic passivation of GaAs surfaces through the formation of arsenic-sulfur bonds" appearing in Applied Physics Letters, volume 54, 1989 at pages 362-364. No heat treatment is described in either reference and the method is believed to not provide satisfactory passivation.
Those same two present inventors have filed U.S. patent application, Ser. No. 087,420, on Aug. 21, 1987. This application discloses a passivating sodium hydroxide film for a semiconductor material of In.sub.0.52 Ga.sub.0.47 As. Without the passivating film, the InGaAs showed a surface recombination velocity of .about.5000 cm/s. The sodium hydroxide film reduced the value of S.sub.o to .about.200 cm/s. Although it was observed that S.sub.o increased by a factor of two within a few days, thereafter it remained essentially constant for at least four weeks
Hawrylo et al disclose in U.S. Pat. No. 4,095,011 the surface passivation of a GaAs light emitting diode by a passivating layer of As.sub.2 S.sub.3, As.sub.2 Se.sub.3 or As.sub.2 Te.sub.3. The film was deposited in a diffusion furnace operating near 640.degree. with the As.sub.2 S.sub.3 source a few degrees cooler. These temperatures are well above the melting temperature of 315.degree. C. for As.sub.2 S.sub.3. No experimental results were disclosed. This method does not seem to be further reported in the literature and has not found widespread acceptance.
Fariver et al in a technical article entitled "Influence of the substrate on the electrical properties of As.sub.2 S.sub.3 films" appearing in Soviet Physics, Semiconductors, Vol. 19, No. 7, 1985 at pages 795-797 investigate the electrical properties of a glassy As.sub.2 S.sub.3 film formed on different types of substrates by vacuum deposition. One of these substrates was n-type GaAs. Their work describes only the electrical characteristics of the As.sub.2 S.sub.3 film and some hypothetical heterojunction characteristics between the As.sub.2 S.sub.3 and the substrate.